This invention relates generally to an electromechanical cable that is especially adapted for antitorsional deployment along a generally straight trajectory.
In dealing with electromechanical cable there is a close interdependence among the method of manufacture and the cost thereof; the method of packaging and transporting the product; the method of deploying the product in the field; and the requirements imposed upon the cable when it is in use in terms of electrical, mechanical and chemical characteristics. In manufacturing the cable it is necessary that it be wound upon a drum, or into a coil, or the like, in order to later transport it to its location of use. The cable is then unwound from its drum or coil in conjunction with its deployment. Thus the attributes of the cable which may be desired for purpose of its use are limited not only by the available techniques of manufacture and the costs associated therewith, but also by the engineering design requirements and criteria for winding up or coiling the cable in the first instance as well as for unwinding or uncoiling it when it is subsequently deployed.
Where electrical cables extend for short distances or are fully supported, the basic requirement is simply to provide a coating of insulation around each conductor wire. This type of construction is used extensively, for example, in household extension cords for appliances, telephone instruments, and the like.
Some types of construction involve long lengths of cable in which, nevertheless, the cable is well protected. This is true when the cable is placed inside a rigid metal conduit, or buried in a ditch or trench, or laid within an underground tunnel. Such applications impose little in the way of longitudinal tensile load upon the cable, and also require little in the way of "armoring" for the cable to protect itself from its environment.
But in other construction situations the requirements may become severe, and these requirements are discussed under the following headings:
A. Longitudinal support. The cable may be suspended for great distances, either horizontally or vertically, either in air or in water. A considerable longitudinal tensile stress is placed upon the cable in order for it to support itself. Since the conductors are generally made of copper or other soft metal, it is necessary to provide in addition to the conductors one or more members having a high tensile load capacity and which may be referred to as "strain members". The strain members are often made of steel but also may be made of various other materials.
B. Armoring. Mechanical protection of the cable from its environment may require full or partial surrounding of the conductors with a mechanical structure which will resist a cutting or piercing action from the outside of the cable, or a bursting action of the cable itself. The "armoring elements" of a cable may be identical to the "strain members", or may be largely separate from them, or perhaps totally separate from them.
C. Chemical Protection. In any type of environment to which the cable is subjected it will require some degree of chemical protection. This requirement is perhaps greatest for submarine cables that are submerged in the ocean and continuously exposed to salt water and to ocean organisms.
D. Freedom from Torsion. The breakage of electrical cables resulting from kinkage has often been a problem. It has been well-known that the variation in the longitudinal tensile load upon a cable, whether it be an increase or decrease in the stress, may tend to cause the cable to twist in one direction or the other. Hence it is desirable to construct the cable in such a way as to avoid this problem.
For transportation and deployment, the method in common usage has been to wind the cable upon a drum or spool at the factory, and then transport the loaded drum to the place of deployment. This procedure involves rotating the drum about its axis in the first instance when the cable is being wound upon it, and then rotating it about its axis again at the deployment site. Depending upon the diameter and length of the cable, the loaded drum can become very bulky and heavy. Deployment of the cable then requires a considerable amount of power, and is also very slow and time-consuming because it is not feasible to rotate the loaded drum rapidly.
It has long been known that one way to make a cable easier to bend, and hence easier to wind upon a drum, is to construct it with armor or strain members all of which are helically wound about the cable in the same direction. The winding of the cable upon the drum stretches the strain members at the outer surface of the cable while compressing them on the inner surface of the cable, but because of the helical winding each strain member lies on the inner and outer surfaces alternately, with the result that the stretching and compressing forces offset each other. While such a cable construction is very satisfactory for purposes of transporting and deploying the cable, it does not usually meet the requirements for the end use of the product. The difficulty is that the helical wind of the strain members produces a torsional effect. Every time longitudinal tension on the cable increases, or the cable becomes slack, it tends to twist, and may then kink and break.
In the construction of submarine cables it is well-known to place the electrical conductors in the interior portion of the cable and surround them with metal armoring elements which provide both mechanical protection and longitudinal tensile support. These armoring elements are commonly constructed in two approximately equal portions which are helically wound in opposite directions. When the longitudinal tensile stress upon the cable varies, the torsional force developed from armoring elements wound in one direction is offset by the torsional force developed from those wound in the other direction. The technique of balancing the twisting forces is described, for example, in U.S. Pat. No. 3,374,619. Cables constructed in this fashion have no significant tendency to twist up and kink. They are deployed by unwinding from a loaded drum which is rotated about its axis.
In the prior art there has been a known technique for deploying a wire or cable without the necessity of rotating the coil or roll. To achieve this result the wire or cable must be pretwisted as it is being wound into the coil or roll. During deployment, pulling the wire or cable out straight from the stationary coil relieves the pretwist, so that the wire or cable is deployed in a straight and untwisted condition. This technique is described, for example, in U.S. Pat. No. 2,709,553 to Wellcome. However, it has not been known to apply this method to electromechanical cables that contain high tensile strain members as well as conductors.
The problem to which the present invention is directed is to deploy a cable into the ocean by pulling it from a small, relatively stationary coil or pack, and yet after the cable is deployed to have it be free of torsional effects. Thus, the cable in use must not have a tendency to twist, kink or break as a result of changes in the longitudinal tensile load on the cable.
Among the relevant prior art patents are U.S. Pat. No. 3,006,792 to R. Monelli and U.S. Pat. No. 3,115,542 to G. Palandri et al.